I've done the Vintage audio thing for some time and new audio as well. what i can say is its dependent on the specific item your looking at in vintage. Not all are as good as others. Also the technology of class A, A/B hasn't changed all that much in 40 years obviously the newer classes are a change i.e. class D etc.
The usual question is what has the older piece had done to it for maintenance over the years. All components drift out of spec over time so if that component has been refreshed/recapped etc. then yes the item can compete fairly well with modern designs of similar class type. Specially if the original specs were excellent to start with. Material science has come a long way in the last 40 years though so that has to be taken into consideration as well.
I have older quality vintage pieces that compete very well with modern equipment.